Not Applicable.
1. Field of the Invention
The present invention relates in general to video telephony, and, more specifically, to improving the perceived quality of a video telephony call when separate video and voice channels are used to establish the video telephony call.
2. Description of the Related Art
Internetworking (i.e., the interconnection of many computer networks) allows the interaction of very large numbers of computers and computer users. The most well known example is the Internet. As the number of users with access to the Internet has grown, so has the use of the Internet as a tool of direct person-to-person communications. Examples include IP telephony (also referred to as Voice-Over IP or VoIP) and Internet video conferencing (e.g., Microsoft NetMeeting).
Related U.S. applications Ser. No.09/978,616 and Ser. No. 10/033,813 teach a video telephony system wherein the Internet or other computer network provides a channel for carrying a video portion of a video telephony call while the POTS (plain old telephone system) or PSTN (public switched telephone network) provides a channel for carrying the voice portion of the video telephony call.
An important issue in Internet communications is the bandwidth or speed at which any particular connection operates. In the case of prior art video conferencing using the Internet, insufficiency of the bandwidth utilized for a video call has caused poor voice and picture quality. In the above video telephony system, the voice channel provides more than enough bandwidth to ensure that a good quality voice transmission is obtained. In addition, removing the voice data from the Internet transmission frees up more of the available bandwidth for the video data in that channel. Moreover, since the actual understanding of the video telephony conversations by the participants depends more on the voice signals than on the video signals, the overall satisfaction with video telephony is increased even when video quality may be somewhat lacking.
Another important issue related to bandwidth is network latency, which is the delay between when a signal is sent by the sender and when it is received by the recipient. Such delays during a two-way communication can cause unnatural conversation. In the system of the present invention wherein voice signals and video signals are delivered by separate communication channels, the further potential exists for reception of the signals to become unsynchronized.
The present invention has the advantage that the voice and video presentation at the receiving end maintains synchronization by adapting to current latency conditions.
In one aspect of the invention, a method of synchronizing reproduction of voice signals and video signals in a video telephony call comprises transmitting the voice signals from a first telephone device of a first party to a second telephone device of a second party via a public switched telephone network (PSTN) connection. The video signals are transmitted from a first computer of the first party to a second computer of the second party via packets in a computer data network. The video signals comprise successive video frames. A video latency time is determined for transmission of the video signals over the computer data network. If the video latency time is in a first predetermined range then transmission of the voice signals is delayed from the first telephone device to the second telephone device by a selected delay in response to the video latency time. The selected delay is no greater than a predetermined maximum voice delay.